The HBT effect in interferometry was something unexpected. When Hanbury, Brown and Twiss published results claiming to have measured the angular diameter of Sirius using what amounts to entangled photons, they had more than a few detractors. This from the wikipedia page Cut to the chase, nowadays cosmological HBT interferometry is trying to answer questions about inflation and fluctuations in the CMB. So what is it? Any takers?
Tag, arfa. Ze HBT-effect appears to be a very interesting phenomenon. I've never heard of it, however, Prof. Silverman has, so I will let him explain it to us. Here is his reply:
I'm guessing the real OP issue is that the strange statistical 'photon bunching' of ostensibly random streams implied in HBT effect really implies there is interference between photons. Something Dirac declared never happened 'because it would violate conservation of energy': http://skullsinthestars.com/2008/09/12/interference-between-different-photons-never-occurs-not-1963/ - but as shown later by others, interference between photons does happen. See also pp858-859 here: http://aflb.ensmp.fr/AFLB-295/aflb295m308.pdf Some time back I introduced a simple scenario that implied coherent photons always interfere with each other. I got no sensible response from QM experts until one SF member, for the first time ever, offered a QED perspective that seemed to reconcile my scenario with 'photons only self-interfere'. In hindsight, it never really made full sense and imo implied a runaway process massively violating conservation of energy.
Mkay. So here's another thought experiment. Use a metal plate with a circular hole in it as a diffraction grating. Have a heater around the aperture and investigate the effect of temperature on photons initially, then other bosons/fermions whose wavelength is much smaller than the aperture diameter. And of course, use an HBT interferometer. And thanks, tashja, Q-reeus for the posts. I looked up Mark Silverman in an online university catalog and he's quite the published scientist. Q-reeus I think that second link looks like a good paper, it seems to confirm my suspicions that they really are saying photons from different parts of a star are entangled with the distance between them when they leave the surface.
In the case of just source photons, what is to be anticipated, other than heater supplying an additional source of MB distributed radiation? Is there a credible mechanism for entanglement there given a lack of common origins between regions? In any case surely one expects instantaneous disruption of any fleeting initial entanglements in such an environment - cf difficulties in maintaining entanglement in quantum computing. My initial guess would have been fleeting patches of superradiance are playing a role at the source end of things: https://en.wikipedia.org/wiki/Superradiance However, that quote of Purcell in the second linked-to article I gave earlier, makes it clear HBT is really a subtle multi-photon interference phenomenon. Dirac can turn in his grave!
This technique has been used for detection of gravitational lensing for some time. Multiple images of objects being lensed can be sorted out this way. Unless I am mistaken, this technique is part of the complement of the last instrument upgrade to Hubble. A good book to read on the impact of this technique is Evalyn Gates' 'Einstein's Telescope'.
I would agree. https://en.wikipedia.org/wiki/Hanbury_Brown_and_Twiss_effect#Quantum_interpretation https://books.google.com/books?id=B8xGBQAAQBAJ&lpg=PP1&pg=PA197 (Ed Purcell, 1956)
Well, interference and entanglement are kind of the same thing, or entanglement can be considered as interference between probabilities that particles are in correlated states. It's also because the pairs of photons from the star are 'entangled' with paths to both detectors. You can't detect which path each photon takes. This is what the authors say in the introduction, from your second link: As to the thought experiment, you arrange a beam of particles with a larger diameter than the circular aperture so you get edge diffraction. The distance to the detectors is another variable.
This actually hits remarkably close to the problems I'm grappling with at work right now. The thing that finally helped me wrap my head around it a little was the distinction between "particle entanglement" and "mode entanglement". Very briefly, a photon is an excitation of the electromagnetic field, while its mode is a description of what sort of excitation it is. Polarization, frequency, and spatial location all factor into the description of a photon's mode. Two photon detectors at spatially distant locations, then, are basically looking for photons in orthogonal modes. If we consider an HBT-type interferometer in which there are two modes (one for each detector), any quantum state can be written in the basis |n,m>, which denotes n photons in mode 1 and m photons in mode 2. When you detect one photon in each mode, you're detecting the state |1,1> in this notation. |1,1> is a separable (non-entangled) state, which I think is why so many physicists were initially skeptical that HBT interferometry could work. But note that there is another way to write our state: given that we have two photons, |x,y> can instead denote that the first photon is in mode x and the second is in mode y. We've gone from a mode-basis to a particle-basis. As Hasselbach et al. point out in the above quote, photons must be symmetrized because they are bosons; that is, any state in the particle basis has to be symmetric under exchange of photons. To put one photon in each mode while staying symmetric, we must have the state |1,2>+|2,1>, which is maximally entangled. A one-photon-in-each-mode state has no mode entanglement but a lot of particle entanglement, and it's the latter that HBT interferometry harnesses. More broadly, the question of which quantum effects can arise from particle entanglement and which require mode entanglement is a surprisingly subtle one, and the exploration thereof is very much current science.
I had no idea that 'mode' in QM had such a broad meaning - very different to classical EM, where more-or-less by definition orthogonal modes never interact, at least not in linear and isotropic environments. Evidently mode entanglement exists even for a single photon: http://www.researchgate.net/post/Is_entanglement_for_one_single_photon_possible
